In ballistic testing it is desirable to monitor and observe the flight characteristics of fired projectiles as they travel along their trajectories. It is especially important to monitor projectiles at certain preselected locations in close proximity to a gun muzzle. In a wide variety of conventional testing devices, use is made of a fixed time delay generator for detecting the estimated presence of a projectile after a gun is fired. However, such conventional equipment is prone to generate errors at a preselected location due to different velocities of a series of projectles fired from the same gun tube. Such errors increase as the distance to the monitoring location increases from the muzzle.